Darwin's Theory of Evolution by Natural Selection

Phenotypic variation exists among individuals and the variation is heritable.

Those individuals with heritable traits better suited to the environment will survive.

When reproductive isolation occurs new species will form.

These are the basic tenets of evolution by natural selection as defined by Darwin. The following is a quote from Darwin.

"Variation is a feature of natural populations and every population produces more progeny than its environment can manage. The consequences of this overproduction is that those individuals with the best genetic fitness for the environment will produce offspring that can more successfully compete in that environment. Thus the subsequent generation will have a higher representation of these offspring and the population will have evolved."